Analysis of linear circuits

Thread Starter

JojoBee

Joined Jun 1, 2020
4
Hello everybody,

I am just getting into electronics and am working on the analysis of 7 electrical circuits. The asked information are:
  • if the circuit is coherent or not
  • the voltage u and the current i depending on e, j and the resistance(s)
  • if more than 1 power source, determine which one(s) deliver and which one(s) absorb power
(N.B.: i >0 and e > 0 and voltage arrow points towards point of higher voltage)

I have come up with the following solutions for the circuits (shown in the left column). Could you tell me if/where I am wrong and if possible explain how else one would have to go about it? Especially the last circuit I think is not correct, I hope my attempt at an answer and the calculation route I used is clear by my scribbeling.
Any help is greatly appreciated, thank you so much!
Best regards,
Jo

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WBahn

Joined Mar 31, 2012
25,914
You are using a notation that is uncommon in this part of the world, so a lot of members won't have seen it. Also, it is similar to other conventions we've seen and not all of those conventions are compatible. So let's be sure we are all on the same page.

When you use an arrow to indicate a voltage and the voltage is positive, is the tip of the arrow more positive than the tail or the the other way around? Your Figure #1 implies that the head of the arrow points to the more positive side (and that's assuming that you are using conventional current and not "electron flow").

In Figure #3, which side of the resistor is more positive? Is that consistent with your answer?

In Figure #4, if the voltage source does not deliver any power to the circuit, then what happens to the power associated with the voltage across it multiplied by the current through it? What if the voltage source was set to j*R? How much voltage would be across the current source? How much power would it be delivering? Would the voltage source still not be delivering any power to the circuit? What if the voltage source was set to an even higher voltage? What if it's voltage was negative?

In Figure #5, think about your first sentence -- how can the two voltage supplies result in a voltage of 2e if they are both shut off? Again, consider what happens if e is a large positive number? Is zero? Is a large negative number?

In Figure #6, your statement is two general. What would be the problem if the right-hand current source were replaced with a resistor?

For Figure #7, take your answer and assume it is correct. Now determine if that is consistent with the problem. For instance, assume your current result is a given and that you are told that the left-hand source has an output of e but you are tasked with finding the voltage of the other source. Everything else is the same. Do you get 2e as the result?

One of the beautiful things about most engineering problems is that the correctness of the answer to a given problem can usually be confirmed from the answer and the problem itself.
 

Thread Starter

JojoBee

Joined Jun 1, 2020
4
Hi WBahn,

Thank you so much for the fast reply!
You are right, I did not think about the different kinds of notation, my apologies. Thank you anyway for trying to decipher it.
As far as the direction of the voltage arrow is concerned, it is also supposed to point towards the positive side in the given cases.
I tried to incorporate your comments in the following:

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Thanks for any additional feedback! I am still working on the last circuit ;)

Best regards,
Jo
 

WBahn

Joined Mar 31, 2012
25,914
First off, there's no particular reason that you should have been aware that significant parts of the rest of the world use different symbols and notation than you've been taught. It would be nice if everything were nicely standardized, but they aren't. So at some point you just become aware, as you just have, that they aren't and learn to start being sensitive to the need to get on the same page early in a discussion.

You explanations are much, much better. Your last one, however, has issues (and your uncertainty is noted -- by the way, don't be afraid to note uncertainty on something you turn in; you are much more likely to receive partial credit for at least recognizing that your explanation has issues).

If two current sources that supply the same current are in series, then there is no problem. BOTH current sources provide the current j. What is indeterminate is how much voltage appears across each of the sources. This is directly analogous to the case where two ideal voltage sources of the same voltage are connected in parallel -- there's no inherent conflict, but it is indeterminate how much current is being supplied by each of them.
 

Thread Starter

JojoBee

Joined Jun 1, 2020
4
Thank you so much, that helps a lot!

I have been working on the final circuit (I realised several mistakes), but have come across another comprehension diffulty.
Switching off the voltage source 2e worked out fine, the circuit seems coherent:
1591130050968.png

When I switch off the other source, however, I have trouble determining the values in the circuit right away, as I don't know the voltage drop over resistor to calculate the current going through one of the parallel branches (as was the case in the scenario above).
Could you give me a hint where to start? (In my first attempt I made the mistake of using system voltage for this calculation)
Thank you again very much!
Best regards

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Last edited:

WBahn

Joined Mar 31, 2012
25,914
Can you replace the two R resistors on the left with their equivalent?

Can that then be combined with the R/2 resistor on top?

What does the circuit look like then?
 
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